What prevents speciation from occurring in sympatric populations?

As long as organisms living in the same geographic area are able to interbreed freely, and the environmental conditions remain roughly constant, there should be no special selection pressures upon any particular trait (and thus gene) that might cause the gene pool to diverge into distinct populations. In other words, the population should stay roughly the same, and any changes in the gene pool would be due to chance (or possibly behavioural isolation which can theoretically arise within a population).